1. Field of the Invention
The present invention relates to a rolling method and apparatus for manufacturing steel bars used for concrete reinforcement or the like. In particular, the present invention relates to a rolling method by which various steel bars for concrete reinforcement (hereinafter, referred to as concrete-reinforcing steel bars) having high dimensional accuracies, such as knot height can be obtained using the same finishing rolls regardless of the diameters of the steel bars of final products. Concrete-reinforcing steel bars of various types, including non-lib type, 2-lib type, and 4-lib type, can be manufactured. The present invention is particularly suitable for manufacturing concrete-reinforcing steel bars of the 4-lib type.
2. Description of the Related Art
In conventional rolling methods for manufacturing concrete-reinforcing steel bars, a raw material is rolled through a series of rolling stands in an early stage of the process so as to form an oval cross section. After that, the raw material, now in oval form, is rolled with a final finishing rolling stand equipped with a pair of rolls (total of two rolls) so as to form knots on the circumferential surface of the raw material.
FIG. 1 is an elevation view of conventional finishing rolls in a final finishing rolling stand. Each of the finishing rolls R.sub.1 and R.sub.2 has a roll caliber 1 which has a circular arc cross section, one or more grooves 2 to form knots (hereinafter, referred to as knotting round calibers) formed on the roll caliber at a pre-determined pitch in the circumferential direction of the roll and one groove 6 to form a lib (hereinafter, referred to as lib-forming groove) formed on the roll caliber bottom in the circumferential direction of the roll. A raw material S which has been rolled through a aeries of rolling stands in an earlier stage of the rolling process, providing an oval cross section is rolled with this final finishing rolling stand.
FIG. 2 contains schematic drawings showing the shape of one example of a final product 4-lib type steel bar S. The final product, i.e. a round rod material, has knots 3 on its circumferential surface. Additionally, the round rod material has four libs 4 on the surface along the axis direction which are formed due either to overfill of the raw material S from between the rolls R.sub.1 and R.sub.2 or to metal flow to the lib-forming groove. Such a concrete-reinforcing steel bar is specified by:
I unit weight; PA1 II diameter D excluding the heights of the knots 3 and libs 4; and PA1 III design of the knots 3, namely, PA1 rolling a raw material in a final finishing rolling stand with rolls each having a roll caliber and one or more knotting round calibers on the roll caliber at a pre-determined circumferential pitch so as to form knots on the circumferential surface of the raw material, wherein the final finishing rolling stand includes two pairs of rolls each having the roll caliber and one or more knotting round calibers, and wherein the two pairs of rolls are arranged to exert rolling pressures in two orthogonal directions on the raw material.
i) height t of the knots 3, PA2 ii) distance L between knots 3 next to each other in the axis direction, and PA2 iii) distance d between knots 3 next to each other in the circumferential direction. In particular, the tolerance range for the height t of the knots 3 is strictly specified.
In the prior art, when concrete-reinforcing steel bars which have the same knot height but different diameters are manufactured, so that all the steel bars satisfy the strict specifications, the rolls of the rolling stand must be changed from rolls fixed for a first diameter to rolls fixed for a second diameter. If a concrete-reinforcing steel bar having a small diameter is manufactured by rolling with a 2-roll rolling stand fixed for a diameter larger than that of the steel bar being manufactured, the roll gap between finishing rolls R.sub.1 and R.sub.2 in the rolling stand must be changed from a value H.sub.1 to a value H.sub.2 smaller than H.sub.1, as shown in FIG. 8. In this case, the knot height can not be controlled in the direction perpendicular to the direction of the roll gap, and therefore, each knot formed will have an irregularity in height relative to the circumferential direction. As a result, the produced steel bar will not satisfy the required specifications. Further, when a 4-lib type concrete-reinforcing steel bar is manufactured by rolling with a 2-roll rolling stand, breakage frequently occurs in a corner portion formed at the bottom of the caliber of each roll. Accordingly, steady rolling cannot be completed due to wear of the rolls.